The electronics industry employs laser systems to trim thick or thin film resistors to relatively desired resistance values. An article by Albin et al., entitled “Laser Resistance Trimming from the Measurement Point of View,” IEEE Transactions on Parts, Hybrids, and Packaging; Vol. PHP-8, No. 2, June 1972, describes measurement issues and the advantages of using a solid-state laser for trimming thin film resistors. An article by Swenson et al., entitled “Reducing Post Trim Drift of Thin Film Resistors by Optimizing YAG Laser Output Characteristics,” IEEE Transactions on Components, Hybrids, and Manufacturing Technology; December 1978, describes using green (532 nm) solid-state laser Gaussian output for trimming thin film resistors to reduce heat-affected zones (HAZ) and post-trim drift.
U.S. Pat. Nos. 5,569,398, 5,685,995, and 5,808,272 of Sun and Swenson describe the use of nonconventional laser wavelengths, such as 1.3 microns, to trim films or devices to avoid damage to the silicon substrate and/or reduce settling time during passive, functional, or activated laser trimming techniques. U.S. Pat. No. 6,534,743 of Swenson et al. describes the use of a uniform laser spot in a generally ablative nonthermal wavelength to reduce microcracking, HAZ, and shifts in the temperature coefficient of resistance (TCR).
Some resistor trimming techniques employ a measure/predict trim measurement process that measures a resistor's value with the laser not cutting and then predicts how much additional laser trimming should be conducted to reach a desired value. This predictive trim procedure may be performed only once during a resistor trimming operation or repeated several times, and is relatively slow since the resistor must be allowed to stabilize in order to provide the required measurement accuracy.
Some resistor trimming techniques employ a tracking trim or continuous trim process. Under typical tracking trim or continuous trim operations, a current or voltage is applied to the resistor device under test (DUT) and the resistance is monitored while the resistor is being trimmed. Some tracking or analog trimming and measurement processes measure a resistor's value after every pulse. In these techniques, the laser pulsing is stopped as soon as the resistor reaches the desired value. Measure/predict trimming can be more accurate since more time is available to make measurements; however, tracking trim is typically faster, particularly when measurement settling delays are minimized. However, the accuracy of such techniques can be limited when certain transient deviation effects are not considered.
The laser trimming process raises the temperature of the resistor. This added heat affects the measured resistance due to the TCR of the resistor, laser-induced thermal electromotive forces (EMFs), and currents such as those caused by Seebeck and Peltier effects. Fixed offsets in measurement can be typically corrected by using auto-zeroed measurements. Offsets caused by the actual laser trimming are more difficult to correct, particularly for low resistance values. These errors are more difficult to correct because the transient effects caused by heating cannot be practically addressed in a tracking trim process. In addition, the thermal effects in low-value resistors become greater in proportion to the voltages used to measure the resistance across the low-value resistors. These heating effects can be particularly significant in high-gain or critically-balanced circuits and in low-ohm resistors, such as resistors having values less than or equal to 10 ohms. Low-ohm resistors are frequently employed for current sensing applications and as measurement shunts, and may have values of less than or equal to 0.1 ohms.
As the ohmic value of the resistors being trimmed gets lower, induced thermal (i.e., thermocouple) voltages may become larger compared to the ohmic voltage of the resistor. Thermal voltages equivalent to several percent of the voltage developed by 0.2 amps in a 0.1-ohm resistor have been observed.